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Prof. Englund Joins MURI Team Exploring Quantum Memories

Long-Distance Entanglement: Columbia Engineering Joins $8.5 Million Research Initiative to Study Best Approaches for Quantum Memories

Dirk Englund (at right), assistant professor of electrical engineering and of applied physics at Columbia University’s School of Engineering and Applied Science, is part of a seven-university team just awarded a five-year $8.5 million Multidisciplinary University Research Initiative (MURI) by the U.S. Air Force Office of Scientific Research (AFOSR) to develop optimal approaches to create scalable quantum networks.

The team will develop efficient light-matter interfaces for creating entangled quantum memories that could facilitate the long-distance transmission of secure information. The five-year MURI will be led by the Georgia Institute of Technology and include scientists from Columbia Engineering, Harvard University, the Massachusetts Institute of Technology, the University of Michigan, Stanford University, and the University of Wisconsin.

Information is ultimately stored in some kind of physical system.

“In a classical device, this could interchangeably be the charge across a capacitor, the magnetic field orientation on a magnetic hard drive, or currents or light pulses connecting these memories,” says Englund. “For quantum information systems, we similarly want to be able to connect quantum states encoded in lots of different physical systems, such as photons, atoms, and quantum memories in solid state systems. My group is working on one of these solid state systems—electron spins in diamond color centers—and we’re trying to efficiently interface these with photons, using nanophotonic resonators.”

He adds that, once in the optical domain, quantum information could then be transmitted long distances and interfaced with other types of memories.

The MURI scientists will study three different physical platforms for designing the matter-light interaction used to generate the entangled photons. These include neutral atom memories with electronically-excited Rydberg-level interactions, nitrogen-vacancy (NV) defect centers in diamonds, and charged quantum dots.

“We want to develop a set of novel and powerful approaches to quantum networking,” said Alex Kuzmich, a professor in Georgia Tech’s School of Physics and the MURI’s principal investigator.

Overall, the MURI has four major goals:

To implement efficient light-matter interfaces using three different approaches to entanglement;

To realize entanglement lifetimes of more than one second in both the nitrogen-vacancy centers and atomic quantum memories;

To implement two-qubit quantum states within memory nodes;

To integrate different components and physical implementations into small units capable of significant quantum processing tasks.

Quantum memories generated from the interaction of neutral atoms and light now have maximum lifetimes of approximately 200 milliseconds. But improvements beyond memory lifetime will be needed before practical systems can be created.

“We aim to be able to combine systems, so that instead of just one memory entangled with one photon, perhaps we could have four of them,” Kuzmich added. “This may look like a straightforward thing to do, but this is not easy in the laboratory. The improvements must be made at every level, so the difficulty is significant.”

Among the challenges ahead are maintaining separation between the different memory systems, and minimizing loss of light as signals propagate through the optical fiber systems that would be used to transmit entangled photons.

“Light is easily lost, and there’s not much that can be done about that from a fundamental physics standpoint,” said Kuzmich. “The rates of these protocols go down rapidly as you try to scale up the systems.”

The most immediate applications for the quantum memory are in secure communications, in which the entanglement of photons with matter would provide a new form of encryption.

In addition to Kuzmich and Columbia Engineering’s Englund, collaborators in the MURI include:

Luming Duan, professor of physics in the School of Physics at the University of Michigan, Ann Arbor, Michigan.

Marko Lonkar, associate professor of electrical engineering in the School of Engineering and Applied Sciences at Harvard University, Cambridge, Massachusetts.

Brian Kennedy, professor of physics in the School of Physics at the Georgia Institute of Technology, Atlanta, Georgia.

Mark Saffman, professor of physics in the Department of Physics at the University of Wisconsin, Madison, Wisconsin.

Vladan Vuletic, the Lester Wolfe Professor of Physics in the School of Physics at Massachusetts Institute of Technology, Cambridge, Massachusetts.

Thad Walker, professor of physics in the Department of Physics at the University of Wisconsin, Madison, Wisconsin.

“If we are successful with this over the next five years, long-distance quantum communications may become promising for real-world implementation,” Kuzmich added. “Integrating these advances with existing infrastructure – optical fiber that’s in the ground – will continue to be an important engineering challenge.”